Mutants of Streptococcus mutans strain BHT have been isolated which produce decreased lactic acid from fermentable carbon sources, and which have been shown to lack the enzymatic activity L (plus) -lactate dehydrogenase (LDH). These strains are presently being characterized to determine their possible usefulness as effector strains in reducing the incidence of human caries by replacement therapy. Some of the properties of the mutant strains being examined include: determining their ability to colonize and produce caries in experimental animals; testing their ability to compete, within the oral cavity, with wild-type Strep. mutans of the same and different serogroups; determining the nature of the genetic and physiologic defects producing the observed LDH phenotypes; and seeking 2nd step mutations which further reduce acid production by these strains.